Faster propulsion typically requires dramatically greater amounts of power. For hypersonic flight over Mach 5, propulsion methods like plasma-fuel engines will fly faster than what is currently proposed with scramjet technology but will need vast amounts of electrical power. Current methods of generating this power appear to be inadequate. Fuel cells that use oxygen from the air have the potential advantage of high efficiency, but they cannot be used for high-speed operation because they require stopping the air molecules to produce the reactions that generate electricity. At speeds faster than approximately 2000 m/s, the energy cost of stopping the air, which constitutes drag, will exceed the energy that can be gained from conventional chemical reactions.